On Thursday, Argentina’s Navy said it was officially giving up hope of finding the sailors alive and was looking only for wreckage.

Top Argentine officials now believe a blast instantly killed the sailors and sent the vessel to the seafloor. While no one knows for sure what caused the explosion, the batteries are the likeliest culprit, Argentine naval officials and outside experts say.

“It looks like a very possible cause,” said Mike Fabey, a naval expert at Jane’s by IHS Markit, a defense consulting firm. “Battery issues of all types with submersibles are something people have been trying to deal with since submarines first went into the water. Water plus electricity equals fire.”